Over recent years, in various types of radio communication systems, an OFDM (Orthogonal Frequency Division Multiplexing) technique has been employed. The OFDM technique can increase a symbol length by parallel transmission using a plurality of carrier waves, and therefore it is possible to equalize received signals using a simple receiver configuration even in a multipath communication line having frequency selectivity.
In general, in the OFDM technique, a CP (Cyclic Prefix) is provided between OFDM symbols in order to handle a delay in a multipath communication line. FIG. 10 is a schematic diagram illustrating CP addition processing on a transmitting side. On the transmitting side, an OFDM symbol 601 is generated by IFFT (Inverse Fast Fourier Transform). Then, on the transmitting side, an end 602 of the OFDM symbol 601 is copied to generate a CP 603, and the generated CP 603 is added right in front of the OFDM symbol 601.
Usually, a CP length is designed taking into account a spread of delay between multipath communication lines. However, if the spread of delay exceeds the CP length, anterior and posterior OFDM symbols in the time direction interfere with each other, which causes OFDM ISI (Inter Symbol Interference) and ICI (Inter Carrier Interference) in which subcarriers in the frequency direction interfere with each other, which results in receiving characteristic degradation.
The ISI is an interference that is caused by the fact that OFDM symbols disposed before and behind an OFDM symbol subjected to FFT (Fast Fourier Transform) on a receiving side exceed a CP length and then leak into an FFT window. On the other hand, the ICI is an interference between subcarriers that is caused by the fact that a communication path matrix is not diagonalized in FFT because a communication path matrix becomes a non-circulant matrix because a spread of delay exceeds a CP length.
As techniques to solve the above-described problem, receivers disclosed in PTL 1, PTL 2, PTL 3, and NPL 1 are known. For example, in NPL 1, under the environment where there is a spread of delay exceeding a card interval corresponding to a CP length, an ISI replica and an ICI replica are generated using a signal after decoding, and then the ISI replica and the ICI replica are subtracted from a received signal.
FIG. 11 is a diagram illustrating an example of conventional receivers. Specifically, FIG. 11 is a simplified block diagram to illustrate the receiver disclosed in NPL 1. The receiver disclosed in NPL 1 of FIG. 11 includes an ISI elimination unit 701, an ICI elimination unit 702, an optimum detection filtering unit 703, a decoding unit 704, a symbol replica generating unit 705, an ISI replica generating unit 706, and an ICI replica generating unit 707. Hereinafter, configurations of the ICI replica generating unit 707 and the ICI elimination unit 702 that relate to ICI reduction processing will be described.
The ICI replica generating unit 707 generates an ICI replica using a symbol replica generated in the symbol replica generating unit 705 and a channel estimation value of an impulse response from a transmission path between a transmitter and a receiver. Specifically, the ICI replica generating unit 707 calculates an ICI channel matrix by performing matrix multiplication for a non-circulant matrix of a communication path matrix generated from the channel estimation value and a Fourier transform matrix. Next, ICI replica is generated by multiplying the ICI channel matrix and a symbol replica vector together. The ICI replica generating unit 707 outputs the ICI replica to the ICI elimination unit 702.
The ICI elimination unit 702 subtracts the ICI replica from a received signal obtained by eliminating the ISI in the ISI elimination unit 701. The ICI elimination unit 702 outputs the received signal having been subtracted to the optimum detection filtering unit 703. The optimum detection filtering unit 703 executes FFT processing and channel equalization at the same time.
As described above, in ICI elimination processing of the receiver disclosed in NPL 1, an ICI replica is calculated for each subcarrier on a frequency axis, and the ICI replica is subtracted from a received signal.